The present invention is directed to a method and apparatus for measuring the amount of moisture that is associated with a web of moving material. More particularly, the invention is directed toward measurement of water in paper as it is being manufactured by a paper making machine.
U.S. Pats. Nos. 3,641,349 and 3,675,019 both assigned to the assignee of the present application disclose moisture gauges for sheet material being manufactured by a paper making machine utilizing a dual wavelength technique. Normally such a gauge includes an infrared radiation source which emits radiation in two spectral bands. The first band of 1.7 microns impinges upon the paper and the amount of transmission through the paper or reflected from the paper is a function of certain parameters of it. It is known that in the case of a spectral band encompassing 1.7 microns or 1.8 microns that this wavelength region is relatively insensitive to the moisture content of the paper. However, in the spectral band including 1.94 microns lies within the absorption band of the water or moisture contained by the paper, it is relatively sensitive. Thus, the ratio of the two spectral bands is a function of the amount of water in the paper.
As illustrated by the foregoing U.S. Pat. Nos. 3,641,349 and 3,675,019, normally several factors contribute to inaccuracies in measurement. These include electronic drift and effects due to dirty environment from which the moisture gauge operates. Such factors necessitated various standardization and/or calibration schemes. However, even with the foregoing, error was still present in measurement of moisture content.